topic 12

• What is a permanent magnet?


  A magnet that produces its own magnetic field. It is always magnetic.
• What type of magnet is a bar magnet?

  A permanent magnet.
• What can a magnet attract?

  Magnetic materials.
• Which metals are magnetic?

  Iron, Steel, Nickel and Cobalt
• What is a magnetic field?

  The region around a magnet where other magnetic materials can experience a force and where others magnetic materials are attracted to it.
• What are the two ends of a bar magnet?

  The north-seeking pole and the south-seeking pole.

  
• Which poles of two bar magnets placed close to each other attract?

  The north pole of one magnet and the south pole of the other.

  
• Which poles of two bar magnets placed close to each other repel?

  The north pole of one magnet and the north pole of the other.

  OR

  The south pole of one magnet and the south pole of the other.

  
• What is an induced magnet?

  When a magnetic material is placed in a magnet field, it becomes a magnet itself temporarily.
• When does an induced magnet stop becoming magnetic?

  When it is taken out of a magnetic field.
• Give 6 uses of magnets.

  - Electric motors

  - Loudspeakers

  - Microphones

  - Generators

  - Door latches

  - Knife holders
• How can the shape of a magnetic field be found?

  By using plotting compasses. We represent the magnetic field using lines that show how a single north pole would move (from north to south). The field is strongest where the lines are closest together.

  
• What does the direction of a magnetic field caused by a current depend on?

  On the direction of the current. If the current changes direction, so does the direction of the magnetic field.
• What is the shape of a magnetic field around a bar magnet?


  Look at the diagram:

  
• What is magnetic field formed between two flat magnets?

  They form a uniform magnetic field which has the same strength and direction everywhere. Look at the diagram:

  
• Where are the strongest fields in a bar magnet?

  At the poles.
• Where are the weakest field in a bar magnet?

  Furthest away from the poles.
• How are compasses used?


  They help people to find their way as the needle, a very small magnet, always points toward the Earth's North Pole.
• What is the behaviour of compasses used as evidence for?

  That the Earth has a magnetic field, which is similar in shape to the magnetic field of a bar magnet.

  
• What is the magnetic field of two bar magnets repelling each other?

  Look at the two bottom diagrams:

  
• What is the magnetic field of two bar magnets attracting each other?

  Look at the first diagram:

  
• What can cause a magnetic field?

  A current flowing through a wire.
• Explain what practical you can do to show that a current can cause a magnetic field.

  A wire is passed through a piece of card with iron filings on it. When a current flows through the wire, the iron filings form circular patterns on the card. This shows that the current is causing a magnetic field because the iron filings are lining up with the direction of the magnetic field.

  
• What direction does a current go in?

  From positive to negative.

  
• What is the right hand grip rule?

  The right hand grip rule can be used to find the direction of the current and the direction of the magnetic field. Coil the fingers of your right hand as if holding the handlebars of a bike, with your thumb pointing away from your hand. The thumb indicates the direction of the current and the fingers then indicate direction of the current. You need to know whether the current from the wire is pointing towards you (like it's going out of the wire) or if it's pointing away from you (like it's going into the wire).

  
• What does the strength of the magnetic field depend on?

  The strength of the current. The higher the current, the stronger the magnetic field.
• When a magnetic field is created by a wire, where is the magnetic field strongest?

  It is strongest closer to the wire, and gets weaker with increasing distance.
• What is a solenoid?

  A coil of wire with a current flowing through it that acts as a bar magnet.

  
• What is the magnetic field of a solenoid?

  Look at the diagram:

  
• Explain the magnetic field of a solenoid.

  The magnetic field of all different parts of the wire form an overall magnetic field. The fields from individual coils add together to form a very strong field inside the solenoid. Outside the solenoid, the fields from coil tend to cancel out the fields from the other side giving a weaker field outside.

  
• What is an electromagnet?

  A coil of wire with a current flowing through it.
• How can the magnetic field of an electromagnet be made stronger?

  By putting a piece of iron (an iron core) inside the coil. This iron becomes a temporary magnet- it's only magnetic while the field from the electromagnet is affecting it.

  
• What is the motor effect?

  When a current-carrying wire is put between magnetic poles (two other magnets), the magnetic field around the wire interacts with the magnetic field it has been placed in. This means that the wire now experiences a force. If the force was in the downwards direction for example, the wire would move downwards.

  
• What does the force in a motor depend?

  Three factors:

  - Magnetic flux density

  - Current of wire

  - Length of wire
• What does the direction of the force in the motor effect depend on?

  On the direction of the magnetic field and the direction of the current.
• What is Fleming's left hand rule?

  Thumb - Force (movement)

  First finger - Magnetic Field (North to South)

  Second finger - Current (positive to negative)

  
• What can Fleming's Left Hand Rule be used for?

  To predict the direction of the force acting on a wire carrying a current.
• How is rotation caused in electric motors?

  By using the force on a conductor in a magnetic field.
• How are magnets used in doorbells?

  These use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise